AccuEnergy AcuDC 241, User Manual

Page: 53 / 70

45
The receiving device recalculates the CRC value during reception of the message, and com-
pares the calculated value to the actual value it received in the CRC field.
An error will be reported if the two values are not equal. CRC calculation is first started by
preloading the whole 16-bit register to 1’s. The process begins by applying successive 8-bit
bytes of the message to the current contents of the register. Only the eight bits of data in
each character are used for generating the CRC. Start and stop bits, and the parity bit, do
not apply to the CRC.
When generating the CRC, each 8-bit character is exclusive ORed with the register contents.
The result is shifted towards the least signifcant bit (LSB), with a zero filled into the most
signifcant bit (MSB) position. The LSB is extracted and examined, if the LSB equals to 1, the
register is exclusive ORed with a preset, fixed value; if the LSB equals to 0, no action will be
taken. This process is repeated until eight shifts have been performed. After the last (eighth)
shift, the next 8-bit byte is exclusive ORed with the register’s current value, and the process
repeats for eight more shifts as described above. The final contents of the register, after all
the bytes of the message have been applied,the final contents of the register, which should
exchange the high-byte and the low-byte, is the CRC value. When the CRC is appended to
the message, the low-order byte is appended first, followed by the high-order byte.
4.2 Communication Format
This chapter will illustrate the format indicated in Table 4-3 (Hex value).
Addr Fun Data start reg
hi
Data start reg
lo
Data #of
regs hi
Data #of
regs lo
CRC16
hi
CRC16
lo
06H 03H 00H 00H 00H 21H 84H 65H
Table 4-3 Protocol Illustration
Addr: Slave device address